The invention relates to a method for controlling the shrinkage of injection moldings in a cavity in a mold of an injection-molding machine after the operation of filling this cavity with a molten material has ended, with the temperature of the mold being controlled, and to a device used for this purpose.
In known methods used to fill a mold for example with thermoplastics, the filling operation is controlled in such a way that an initial speed-managed phase is followed by a pressure-managed phase which lasts until the end of the filling operation. Toward the end of the speed-managed phase or in the starting section of the pressure-managed phase, the filling situation in which the impression is completely wetted with plasticizable material is reached yet the pressure of the material in the interior of the impression is still relatively low. On account of the continuing movement of an injection plunger or an extruder, the internal pressure in the mold rises, which is associated with a reduction in the specific volume or an increase in the density of the molding material inside the impression. The extent of compression which can be achieved in this way is dependent both on the prevailing temperature and on the level of the pressure which is active and on the characteristic properties of the molding material.
After the supply of molten material to the impression has stopped, the molten material in the gate starts to solidify. This seals the impression, making it impossible for any further molten plastic to be supplied. The temperature in the impression drops until the one-bar isochor is reached. The molding then begins to shrink until the molding has reached room temperature.
The shrinkage of the molding is determined by the pressure and temperature conditions and also in particular by the viscosity of the molten material in the cavity. A significant factor in the shrinkage of the molding is the temperature distribution in the cavity at the end of the filling phase (or from the pressure maximum) until the end of the cycle. A differing shrinkage from cycle to cycle results from the fluctuation of the temperature profile and from the fluctuation of the mold internal pressure profile.
This applies both to single molds and to multi-impression molds. When producing injection moldings of all types (plastic, metal, ceramic, etc), for cost reasons it is often the case that a plurality of moldings are produced simultaneously each cycle (multi-impression mold). In this case, the individual cavities are normally balanced with regard to geometry and gate points to a sufficient extent for the quality of the injection moldings to be as uniform as possible. In reality, however, the shrinkage of the individual injection moldings is always different, and also changes constantly, on account of material fluctuations, temperature fluctuations and resulting fluctuations in viscosity.
It is an object of the present invention to provide a simple method which enables the shrinkage of the molding to be made as uniform as possible both between individual cavities of a multi-impression mold and also from cycle to cycle of an injection-molding operation.